1. Field of the Invention
The present invention relates to an electromagnetic clutch having a specific structure for transmitting a driving force and/or a specific structure for supporting a friction member.
2. Description of the Related Art
An electromagnetic clutch is known as a clutch which is engaged and is disengaged by a magnetic force. In this electromagnetic clutch, a magnetic force is generated by an electromagnetic coil, and an armature (friction plate) and a rotor are attracted with each other by the magnetic force. Thus, a friction force is generated between the rotor and the armature, and a driving force (torque) is thereby transmitted. When an electric current is turned off and is not fed to the electromagnetic coil, the armature is separated from the rotor, and the transmission of the driving force is thereby interrupted.
In the above operation, the armature must be supported so as to be separated from the rotor when the clutch is disengaged. In order to satisfy this requirement, the armature is supported by a plate spring so that the plate spring is deformed by a magnetic attracting force, and so that the armature moves toward the rotor and contacts the rotor when the clutch is engaged. For example, an electromagnetic clutch is disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-016818.
FIG. 6 is a perspective view showing an example of a structure for supporting an armature in a conventional technique. FIG. 6 shows a reference numeral 701 that indicates a fixing member to be fixed to a rotating shaft (not shown in the figure). The fixing member 701 comprises a planar (flanged) fixing surface at a portion that cannot be seen in the figure, and a plate spring 703 is fixed on a fixing surface by rivets 702. An armature 705 is fixed to the plate spring 703 by rivets 704. According to this structure, when the armature 705 is attracted to the back side of the figure by a magnetic force, the plate spring 703 is elastically deformed, and the armature 705 is slightly moved in the axial direction. When the magnetic force is removed, the armature 705 returns to the former position by the springback force of the elastic deformation of the plate spring 703.
The above plate spring 703 is fixed to the armature 705 by the rivets 704 so as to ensure the strength for transmitting a driving force to the rotating shaft. This structure requires many parts and assembly steps, and production costs are therefore high. Specifically, in a method for fixing a plate spring by rivets, special tools are required for fixing the rivets, and the operation is therefore complicated.
An electromagnetic clutch is provided with members at the inside thereof, and the members may rotate when the clutch is operated. In order to design a compact clutch, the rotating members must be disposed so as not to contact other members. Accordingly, the accuracy of positioning of the members in the axial direction of the rotating shaft is important. There may be a case in which a rotation angle-measuring device for measuring a rotation angle and a rotation direction is mounted on the rotating shaft. In this case, in order to maintain measurement accuracy, backlash in the axial direction (thrust gap) should be reduced as much as possible.
In order to reduce the backlash in the axial direction, a thin annular spacer called a “shim” is conventionally used, and the shim is passed through a shaft member. The number of shims is adjusted with respect to each device by hand working so that the backlash is reduced. Such a structure using shims requires a large number of parts, and the number of adjustment operations and assembling operations are increased, whereby the production cost is increased.
Output of an electromagnetic clutch is transmitted to the outside by a pulley, and a driving force is transmitted from an armature to a pulley through a rotating shaft. In this case, in order to transmit a driving force to the rotating shaft having a smaller diameter than those of other parts, and to bring out the driving force from the rotating shaft, gears and parts are required. The gears should be formed with high processing accuracy, and the parts should have a high strength (for example, they should be sintered members), thereby increasing the production cost.